1. Field of the Invention PA1 (a) a center wire, the center wire having a length which is greater then its width; PA1 (b) a plurality of stress measuring devices, the stress measuring devices being attached along the length of the center wire for measuring stress present on the apparatus; PA1 (c) a forming material, the forming material being formed around the center wire and the stress measuring devices; PA1 (d) a plurality of noncenter wires, each noncenter wire having a length which is greater than its width, and being wound around the length of the center wire, the stress measuring devices and the forming material; and PA1 (e) a device for collecting data produced by the stress measuring devices, the device being connected with the stress measuring devices; PA1 wherein the stress placed on the apparatus can be measured at more than one location along the length of the apparatus, and wherein the apparatus is spinnable into a rock mass without damaging the stress measuring devices. PA1 (1) drilling a hole in said structure; PA1 (2) placing an apparatus within said hole; and PA1 (3) bonding said apparatus within said hole to said structure; PA1 wherein the stress placed on said apparatus can be measured at more than one location along the length of said apparatus, and wherein said apparatus is spinnable into a rock mass without damaging said stress measuring devices.
The invention relates generally to an apparatus for providing support to a potentially unstable structure, such as a rock mass or the roof of a coal or other underground mine, and/or for measuring the stress, strain and/or tension load placed on the apparatus when inserted into the potentially unstable structure.
2. Background and Description of Related Art
Cables are strong wire ropes which are used in coal and other mining operations in locations in which safe access may not be guaranteed during mining to support the roof of the mine from the surface. Most cables employed in mining operations are made by winding approximately six to eight small wire cables around a center wire cable, which is called a "kingwire." Typical steel cables generally have a diameter ranging between about 10 and 40 millimeters, generally have a length ranging between about 5 and 50 meters, and are generally used in boreholes with a diameter between about 45 and 90 millimeters. Steel cables are installed in holes drilled or bored into the rock present in the roof of the mine (boreholes, and bonded to the rock with grout, to reinforce the rock mass. The grout, which is usually a cement grout, a cement mortar, or a chemical grout, is pumped into the borehole after the cable has been pushed into the borehole.
A large amount of stress is generally placed upon cables used for support in mining operations, and upon the cable bolts which are used to secure these cables to the rock mass. The measuring of such stress on the cables and cable bolts is important for risk management, so that the risk of partial or complete collapse of the roof of the mine, and the resulting injury or death to mine workers is minimized or preferably eliminated.
"Stress" is an intangible quantity which cannot be measured directly. It is only the manifestation of stress which is measured in, for example, pounds per square inch (psi), and is used to estimate the stress. In most mining and civil engineering applications, strain measuring devices are installed in a borehole. "Strain" is the displacement of a cable as the cable moves a linear distance (generally measured in microinches (10.sup.-3 inch)). "Tension" and "load" are the amount of force placed upon the cable in, for example, pounds per foot (lbf), and may place a strain on the cable.
The monitoring of the roof support (by monitoring the stress placed on the cables and cable bolts when inserted into the roof of a mine) has become of increasing importance as deeper mines and higher productivity requirements have led to more mechanization, and to larger excavations. This has resulted in increasing rates of mining-induced stress. As soon as an excavation has been made, the rock surrounding the excavation begins to move and dilate into the new void. While the dilation cannot be stopped, it can be controlled by measuring the amounts and directions of stresses placed upon the cables and cable bolts inserted into the rock. Because cables and cable bolts are a passive reinforcement system in the rock mass, the load which is measured along the cable develops in response to deformation in the rock mass. Monitoring and warning devices have been developed to warn of relative movement between the rock mass and support members for various support types.
Most of the devices for measuring stress or strain on cables and/or cable bolts inserted into unstable structures, such as those described in U.S. Pat. Nos. 4,388,710, 4,803,888 and 5,284,107, are external devices which are positioned on the outside of the cables.
Hyett, "Development of a New Instrumented Cable Bolt to Monitor Ground Support Loads in Underground Excavations," 13th Mine Operators' Conference, Queen's University, Canada, Sudbury (1977), describes an instrumented cable using Stretch Measurement to Access Reinforcement Tension (SMART) technology. The SMART technology involves fabricating a miniature Multi-Point Borehole Extensometer (MPBX) within a tempered stainless steel tube which replaces the kingwire of a 7-wire strand cable. The displacement of six spring-loaded wires caused by stretch (elongation) of the cable occurs, and applied force or load is measured (in kN) using linear potentiometers, rather than strain gauge technology. Disadvantageously, and unlike the apparatus of the present invention, due to the fragility of the potentiometers, these instrumented cables cannot be spun into a rock mass without severely damaging or destroying the potentiometers. Rather, the cables must be hand-laid into the rock mass, which is time-consuming and labor intensive, and can only be used with concrete (cement) grout.
DeVries et al., "Optical Fiber Sensors for Monitoring Strain on Rebar-Type and Cable-Type Bolts," Proceedings of SPIE--The International Society For Optical Engineering 2446: 236-241 (1995), describes the use of short, gage-length optical fiber sensors (mirror and light reflectivity sensors) which are surface mounted, using an epoxy resin adhesive, inside of a small groove of the kingwire of a 7-strand cable, for the measurement of relative strain in rebar-type and cable-type bolts used in the mining industry to support unstable material, or to keep rock masses together. Disadvantageously, and unlike the apparatus of the present invention, the apparatus described by DeVries et al. only measures strain at one location, rather than at more than one location, on the cable. Further, because optical fiber sensors are quite bulky, it is not possible to win a series of cables around a kingwire having these optical fiber sensors attached thereto.
There is a need for an improved apparatus for measuring the stress placed upon a cable or other support inserted into an unstable structure, such as the roof of a mine, which overcomes the difficulties of the prior art.
The present invention provides an improved apparatus for providing support to an unstable structure, and for measuring the stress placed upon the apparatus, when inserted into the unstable structure to determine whether or not the unstable structure is being subjected to mining-induced stress changes, or to stress changes caused by other factors. The apparatus of the invention measures the value of tension, strain and stress placed thereon at various locations along the length of the apparatus in, for example, areas of reduced clearance, such as boreholes present in a potentially unstable rock mass. The apparatus advantageously measures stress placed thereon at more than one location along the length of the apparatus (at multiple locations), and is sufficiently durable that it can be spun into a rock mass (or other structure) without damaging its components. Thus, the present invention need not be hand-laid. Further, the apparatus can be bonded into the rock mass (or other structure) with a variety of different types of grout, such as cement-based grouts and resin-based grouts. Moreover, because stress measuring devices are present inside, rather than on the outside, of the apparatus, the results produced by these stress measuring devices are less susceptible to environmental conditions, which can adversely affect resistance wire instruments. The apparatus of the invention may be quickly and efficiently installed into rock masses and other potentially unstable structures using routine procedures, and provides a means for improving the reliability of roof support systems in deep mine and other applications.
Additional advantages of the invention are set forth hereinbelow and are shown in the accompanying drawings.